1. Field of the Invention
The invention relates in general to braking systems, and more specifically to controllable braking systems for transportation apparatus which transports passengers between spaced landings.
2. Description of the Prior Art
Escalators are provided with an electrically released, mechanically applied brake capable of stopping an up or down traveling escalator with any load up to brake design load.
The maximum braking effort is required to stop a fully loaded escalator going down, and thus the brake is sized accordingly. For example, the brake torque is selected to provide some minimum value of deceleration, such as about 1 or 2 ft/sec.sup.2, when an escalator with rated load is stopped while transporting passengers from an upper landing to a lower landing. Thus, any other condition than a fully loaded escalator going down will result in a higher rate of deceleration. The highest rate of deceleration would occur when a fully loaded escalator is braked to a stop while transporting passengers from the lower landing to the upper landing. This may be about 8 to 10 ft/sec.sup.2, for a typical escalator with a fixed braking torque. The prior art has disclosed many different arrangements which adjust the braking effort, in order to decrease the range between the minimum and maximum rates of deceleration which may occur, by taking such things as speed, load, and/or travel direction into account. For example, the braking effort may be adjusted according to speed, such as in response to an error signal which is responsive to the difference between the actual speed and the desired speed of the escalator while braking to a stop.
The braking effort may be adjusted according to travel direction. When the escalator is started in a predetermined travel direction, the control automatically selects a braking effort associated with this selected travel direction.
The braking effort may be adjusted according to passenger load on the escalator. Certain prior art arrangements sense passenger load via mechanical arrangements which sense speed or deceleration during braking, and make mechanical adjustments which adjust the braking effort accordingly. Still another prior art arrangement senses load by sensing the tensile stress in the drive chain.
A speed feedback system during braking which electrically or mechanically adjusts braking effort in response to deceleration is very costly. A system which brakes in response to travel direction may have wide variations from no load to full load. A system which brakes in response to passenger load sensed by stress in the conveyor drive chain only gives an accurate signal when the travel direction is also factored into the adjustment.